Steam reforming of hydrocarbons



United States Patent Claims priority, application Great Britain Aug. 27, 1959 6 Claims. c1, 48-214) The invention relates to steam reforming of hydrocarbons.

At elevated temperatures, at atmosphericor higher pres sure, and in the presence of a catalyst, hydrocarbons react with steam to give a gas comprising hydrogen, oxides of carbon and methane, the'composition of the gas depending on the conditions under which the reaction is effected. Such a process finds, many applications in industry, such as for example the production of methanol synthesis gas from a distillation fraction of petroleum, and is commonly known as steam reforming of hydrocarbons. The process is usually effected above 600 C., a suitable catalyst being nickel.

Hydrocarbons derived from petroleum invariably contain sulphur containing impurities. Using such impure hydrocarbons in a steam reforming process would result in poisoning of the catalyst; and therefore petroleum fractions are substantially completely desulphurised before being steam reformed.

The relationships between the steam and the constituents of the mixture of carbon dioxide, carbon monoxide, hydrogen and methane produced by steam reforming of hydrocarbons are determined by the chemical equilibria between them, i.e. the Water :gas shift equilibrium and the methane steam equilibrium The supported nickel catalysts commonly used for steam reforming are capable of efiecting a close approach to both these equilibria.

If the amount of steam present is low enough, two further equilibria may be approached, namely:

2CO=CO +C and CH :2H +C Both these reactions result in the formation of carbon. Not only is the presence of carbon undesirable in the product gas, but its deposition causes choking of the cataly st tubes in which the steam reforming reaction is carried out. From the thermodynamics of the system it can be calculated that carbon is not an equilibrium product when the steam ratio is two or more molecules of steam per atom of carbon in the hydrocarbon reactant. Never-.

theless in practice carbon deposition occurs when the steam ratio is considerably greater than two molecules of steam per atom of carbon; and it is found that in order to eliminate carbon deposition the steam ratio may have to be at least 6.

Light distillate (-by which is meant a straight run petroleum distillation fraction having a final boiling point less than 200 '0.) contains such an amount of sulphur that if it were used in a process of steam reforming, the reforming catalyst would soon become poisoned. Hence light distill-ate has always been substantially completely desulphurised before steam reforming it; and a steam ratio of 6 to 7 molecules of steam per atom of carbon must generally be used to avoid carbon deposition. However, we have made the surprising discovery thatif only a small amount as hereinafter defined of sulphur is present in the light distillate, the steam reforming process may be 2 V accomplished, without carbon-deposition occurring, at a steam ratio considerably less than 6.

According to the present invention, therefore, there is provided a process for steam'reforming of hydrocarbons, comprising reaction between a hydrocarbon, or mixture of hydrocarbons, and steam at elevated temperature, at atmospheric or higher pressure, and in the presence of a suitable catalyst, characterised in that thereis present in the reactant hydrocarbon or mixture of hydrocarbons a sulphur containing impurity to such an extent that the said reactant contains between about ,1 and 10 p.p.m. of sulphur by weight.

In practice the steam reforming process is efiected at a temperature between 600 C. and 1000 C. (preferably between 700 C. and 900 C.) and at a pressure between 1 and 20 atmospherea prefer-ably between 10 and 15 atmospheres, absolute. A particularly suitable catalyst is contain between 1 and 5 ppm. of sulphur byweight.

A particularly preferred concentration of sulphur is about 2'p.p.m. by weight. Above 8 ppm. of sulphur by weight it becomes necessary to increase the temperature of operation quite considerably, which is economically a disadvantage.

In order to obtain a reactant containing the requisite amount of sulphur, the initial desulphurisationto which it is necessary to submit the impure hydrocarbon may be carried out under conditions such that the appropriate amount of sulphur is allowed to escape removal. Alternatively an appropriate amount of sulphur containing material, for example the unpu-rified hydrocarbon, may be added to a quantity of substantially completely desulphurised hydrocarbon.

We have found that light distillate containing about 2 ppm. of sulphur by weight can be steam reformed to produce methanol synthesis gas, without deposition of carbon occurring, when the steam ratio used in the process is as low as 2.5 molecules of steam per atom of carbon.

It will be appreciated that the composition of the gas produced by the process is dependent on the reaction conditions, such as temperature, and nature of catalyst.

Example 1 40 l. of light'distillate of boiling range 40 C. to C. were mixed with an equimolecular amountof hydrogen, and the resulting mixture was passed at a pressure of 300 lb./in. through a bed of Zinc oxide, then through a bed comprising a mixture of oxides of cobalt and molybdenum supported on alumina, [and finally through a bed of zinc oxide, all three beds being heated to 380 C. to 400 C.

The product obtained, which was substantially entirely desulphurised, was mixed with 108 kgm. of steam to which had been added 250 ml. of unpurified light distillate containing 300 ppm. of organically combined sulphur by weight. Thus the mixture produced contained about 1.9 ppm. .of sulphur by weight, and the steam ratio was about 3.0 molecules of steam per atom of carbon. This mixture was passed at a pressure of 180 lb./in. through a tube filled with a reforming catalyst containing 18% by weight of nickel. The wall of the tube, which had a diameter of 4.in. and a heated length of 20 feet, was maintained at 860 C. by a surrounding furnace.

N increase in pressure loss across the tube occurred over a period of five days; and during this time no carbon was deposited on the catalyst, no particulate carbon was carried through the tube with the product gas, and no apparent deterioration of the catalyst occurred.

A similar experiment Was performed, using substantially completely desulphurised light distillate to which no sulphur had been added. Carbon deposition was continuous, and complete blockage of the tube occurred in 'less than 24 hours.

' Example 2 A second run was conducted in exactly similar manner to that described in the second paragraph of Example 1, except that the mixture of purified and unpurified light distillate was prepared so as to contain approximately 4 p.p.m. of sulphur by weight. After five days operation carbon formation had not occurred and the activity of the catalyst was in no way diminished. When, under the same conditions, :the concentration of sulphur was increased to 8 p.p.m. by weight there was no detectable carbon formation but there was evident loss of catalytic activity because it become necessary to increase the operating temperature by 20 C. in order to achieve the same,

added sulphur being in the term of organically combined sulphur occurring in a petroleum hydrocarbon.

2. A process according to claim 1 in which the sulphur content is maintained within the range from about 1 to about 5 p.p.m. :by weight.

3. A process according to claim 1 in which the hydrocarbon is a liquid straight run petroleum distillation fraction.

4. A process according to claim 1 in which the hydrocarbon to which the said controlled amount of sulphur is added has previously been substantially completely desulphurized.

5. Process as claimed tin-claim 1 in which the controlled amount of sulphur is introduced by mixing with said hydrocarbon, a hydrocarbon having a substantially higher concentration of sulphur containing impurities.

6. Process as claimed in'claim 1 in which the catalyst is substantially free from silica.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Sachanen: Chen1ical Constituents of Petroleum, pages 24-25 1945 Reinhold Publ. 00., NY. Copy in U.S.P.O; Sci. Library and Div. 31. 

1. A PROCESS FOR STEAM REFORMING OF HYDROCARBONS COMPRISING REACTING A LIQUEFIED PETROLEUM HYDROCARBON BOILING BELOW 200*C. AT ATMOSPHERIC PRESSURE, AND STREAM, OVER A NICKEL STRAM REFORMING CATALYST, AND STREAM, OVER A STREAM RATIO OOF FROM ABOUT 2.5 TO 6 MOLES OF STEAM PER ATOM OF CARBON, AND WHEREIN A CONTROLLED AMOUNT OF SULPHUR IS ADDED TO THE HYDROCARBON SO AS TO MAINTAIN THE SULPHUR CONTENT OF THE OVERALL HYDROCARBON FEED AT A RANGE OF FROM ABOUT 1 TO ABOUT 10 P.P.M. BY WEIGHT, SAID ADDED SULPHUR BEING IN THE FORM OF ORGANICALLY COMBINED SULPHUR OCCURRING IN A PETROLEUM HYDROCARBON. 